"Once in 50 years" bellowed the publicist to the crowd. "I want you to wave at him as he goes out! Wave goodbye to him as he goes out!"

He was talking about a mouse.

The moust was standing on the back of a bunting-bedecked train and blowing kisses to people who, without any coaxing from the publicist, blew them right back again. "Have a good trip" some shouted. "Be a good boy!" trilled someone else.

They were yelling to a mouse.

Of course it was the mouse of the hour: Mickey, or a very short actor dressed up like Mickey, waving goodbye to Los Angeles from a private car with velvet curtains at the end of an Amtrak train. Mouse, 50, is off on a 58-city whistlestop tour to celebrate his his half-century on his earth, to promote the cause of Walt Disney Productions generally, and to plug a 90-minute NBC special, "Mickey's 50th Anniversary," to be seen Sunday, Nov. 19 on "The Wonderful World of Disney."

NBC officially lists the guest stars for the show in alphabetical order, like this: ". . . Betty Davis, Donald Duck, Ella Fitzgerald, Jodie Foster, Goofy, Bob Hope, Elton John, Minnie Mouse," and so on.

"That mouse could buy and sell us all," grumbled one surly misanthrope at the party, but he was clearly voicing a minority view. Most of the 400 people's who braved L.A.'s art deco Union Station on a chilly, drizzly Monday night were caught up completely in the spirit of the thing, whatever the spirit of the thing was. It was a little like seeing Mr. Lincoln off.

It was like seeing the King Tut exhibit pulling out on tour, except it was America's Tut and he's still in office.

From here the train went on to Pasedena, Pomona, Flagstaff, and the 55 other stops on the way with an illuminated yellow shield of the Mouse beaming from the train's rear end. The train will arrive in Washington on Friday morning so that Mickey Mouse can host a birthday party for handicapped children that Amy Carter is giving at the White House.

He will also visit the Library of Congress and attend a film tribute at the American Film Institute theater. In New York later, Mouse will help launch a six-week retrospective of this movie career at the Museum of Modern Art and visit what was once the Colony Theater, which gets the honor of being called his birthplace, because "Steamboat Willie," his first film, premiered there on Nov. 18, 1928.

If you were also born that year, you can get into Disneyland free for the next few weeks as part of the celebration. Listen, this is one mouse who doesn't forget the little people who made him what he is today.

And what is he today? Icon, demigod, demimouse, touchstone, patriarch, and corporate symbol for Disney enterprises and its jillion-dollar operations. But although the Mouse gave no outward signs on Monday, all is not well in the empire that Unca Walt built.

"Wonderful World of Disney" hasn't exactly been getting wonderful ratings. NBC recently celebrated 25 years of Disney TV shows (the first, "Disneyland," began on ABC) but one of the most ballyhooed attractions, the feature-length cartoon "Dumbo" fell flat on its nose in the crucial Nielsens. To date this season, Disney has averaged only a fair to middling 30 percent share, ranking 32nd of 74 rated shows. Its furrowed-browed competition on CBS, "Sixty Minutes," is ranked 15th with an average 37 percent share of the Sunday night viewing audience.

In hopes of boosting those ratings. NBC executives have continually implored the fanatically frugal Disney organization to release more of the studio's classic films to television - the big belly whompers, that is, and not "The Gnome-Mobile" which did a recent crash dive on the network. For many years the Disney TV show had been used largely as an instrument to promote current films in release and as a dumping ground for less treasured old material. New productions made for television do not fare so well; audiences tend to be larger when such beloved old scraps of paper as Donald Duck and Jiminy Cricket are trotted out to do their stuff.

In addition to the shaky network ratings, Disney failed resoundingly with syndicated shows like "The Mouse Factory" and a resurrected "Mickey Mouse Club" in recent years.

It began to look as though the old mouse could sell anything with his kisser on it , except himself. What could it be? A loss of innocence? Probably not. For one thing, the Mouse lost his innocence a long tim ago. He became a corporate logo. You even see his domino ears on checks sent out by the company. His very omnipresence may have hardened the soft spot we are all supposed to have in out hearts for him.

And yet Monday night the old Mousekemania resurfaced in full force. It could be that Mickey's crosscountry tour will awaken a sleeping constituency. His status as a contender should not be underestimated. At Union Station mere passengers who'd arrived to take normal trains looked slightly panic-stricken by the display but invited children's faces brightened on cue when the Mouse finally waded through the mob of cocktail sippers and meatball munchers in a section of the train station roped off with floating red, white and blue balloons.

The creature walked among them in the kind of oversized yellow shoes you'd expect to see on Bette midler, shaking hands, being relentlessly kissed by children, and saying nothing. True, Henry Winkler had failed to appear at the party and the biggest celebrity on hand was probably veteran actor Hans Concried, but a resolute frivolity still prevailed.

As 7:30 approached, the guests made their way out of the station and onto the platform where the train and a high schol band were waiting. The atmosphere was like that of a crowd awaiting a glimpse of a celebrated statesman, which indeed they were.

"He's coming! He's coming!" a little girl began to squeal, jumping up and down and almost losing her balloons. The band struck up "It's a Small World, After All," movement could be discerned in the throng, and the Mouse reappeared to climb onto the train and begin a long round of kiss-blowing. He was accompained by, a Disney spokesman said, a "handler," a four-piece oom-pah band, not one but two publicists, and Mr. and Mrs. Ward Kimball, he being one of the veteran animators in the company. He can remember when it was a shop and not an empire.

Minnie Mouse, whose return address had been on all the invitations, was reported to be back at Disneyland, minding the homestead, but a genuine starlet, adolescent actress Jodie Foster, rushed forward and onto the train in order to give Mickey Mouse a farewell kiss. Then steam began to billow up around the train - just like in "Anna Karenina" - the band struck up the Mickey Mouse Club theme song, a conductor actually shouted "Board!" and the train began to move. An attractive young woman waved good-bye with a class of scotch and soon the yellow shield shrank down to a tiny button in the distance. The Mouse was gone. The party was over.